This invention relates to a tandem type color printer having a plurality of printing units corresponding to colors and, in particular, to a tandem type color printer using bitmap data memorized in a memory to make plural copies of a printed matter.
A conventional tandem type color printer comprises four printing units located in line with predetermined intervals between each other on a conveyance route to print a color image or picture on a paper conveyed on the conveyance route. The printing units are, for example, electrophotographic printing units which has the same structure but which uses different color toners different from one another in color. For instance, the printing units are laser or LED array printing units and use yellow, magenta, cyan, and black toners, respectively. In this case, the printing units successively print a yellow constituent, a magenta constituent, a cyan constituent, and a black constituent of the color picture on the paper in synchronism with conveyance of the paper conveyed on the conveyance route.
The tandem type color printer further comprises a printer controller connected to a host computer to receive a printing request signal and a printing data set representative of the color picture from the host computer. When the printing data set is received, the printer controller converts it into a bitmap data including a yellow bitmap data set, a magenta bitmap data set, a cyan bitmap data set, and a black bitmap data set. Each of bitmap data sets consists of data lines corresponding to scanning lines of the color picture.
A head controller is connected between the printer controller and the printing units to transmit the bitmap data sets from the printer controller to the printing units. That is, the head controller supplies the yellow, the magenta, the cyan, and the black bitmap data sets to the yellow, the magenta, the cyan, the block printing unit, respectively in response to vertical synchronized signals and horizontal synchronized signals. For instance, the head controller supplies the data lines of the yellow bitmap data set to the yellow bitmap data set line by line in response to the horizontal synchronized signals for the yellow on condition that the vertical synchronized signal for the yellow is received.
Each of the printing units has a head and photosensitive drum in an exposing portion. The head is, for example, an LED array and exposes the photosensitive drum charged with electricity in response to the bitmap data set supplied from the head controller. As a result, a latent image is formed on a peripheral surface of the photosensitive drum. Next, the photosensitive drum is deposited by the use of the color toners to change the latent image into a color toner image. Then, the color toner image is transferred on the paper when the paper reaches the printing unit. After the four printing units transfer the color toner images on the paper one after another, the transferred images are fixed on the paper to form a printed image. Thus, the conventional tandem type color printer makes the printed matter with the printed image.
Such conventional tandem type color printer is disclosed in Japanese Unexamined Patent Publication (JP-A) No. 11-143163.
Generally, the printer controller supplies the bitmap data sets to not only to the head controller but also to a memory. The bitmap data sets are memorized in the memory and used for making plural copies of the printed matter. The memory may be a SDRAM (Static Dynamic Random Access Memory) or a hard disk apparatus.
The memory has a plurality of memorizing areas to separately store the yellow bitmap data set, the magenta bitmap data set, the cyan bitmap data set, and the black bitmap data set. In other words, the data lines of each bitmap data set are stored in an area having a plurality of continuous addresses.
To make the second or later copy of the printed matter, the printer controller reads out the yellow bitmap data set, the magenta bitmap data set, the cyan bitmap data set, and the black bitmap data set from the memory. In this time, the printer controller repeatedly selects one of the bitmap data sets by turns and read out one of the data lines of the selected bitmap data set during most of operation. Accordingly, the printer controller must access the memorizing areas arranged apart from one another by turns and takes a long time for reading the bitmap data sets. That is, the conventional tandem type color printer has a problem that a long time is necessary to read out the bitmap data sets from the memory. This is because change of the banks must be carried out in the SDRAM or a magnetic head must move for a long distance in the hard disk apparatus. In addition, the conventional tandem type color printer has another problem that a bus is occupied by the bitmap data sets read out from the memory.